Diverse xenobiotic environmental exposures introduce deleterious stress in living cells. DNA damage response (DDR) counteracts the effects of omnipresent genotoxic insult from within and outside cell. The recruitment of an ever-increasing list of factors to damaged genomic sites is not only intricately and inextricably linked but their interplay dictates the nature as well as course of DDR. Due to a wide-ranging inter-molecular crosstalk between DDR components this continuation grant will focus on studying the interaction and influence of relevant overlapping factors of NER and checkpoint signaling pathways. The proposal is based on the premise that UV damage simultaneously activates diverse events impinging on access to damage and repair as well as restoration of epigenetically intact chromatin and normal cell cycling. Specific hypothesis underlying the proposed work is that initial sensors of UV damage, DDB and XPC complexes, are intimately associated with signaling kinases, ATR and ATM, and their interaction, in conjunction with chromatin remodeling factors, histone chaperons and histone modifying proteins, determines all key aspects of DDR related to NER. The proposed work will utilize a relevant plethora of state-of-the art technologies to address following inter-related specific objectives: (1) to demonstrate the function of DDB and XPC in checkpoint activation, (2) to understand the roles of histone ubiquitination and acetylation in checkpoint signaling, (3) to ascertain the influence of chromatin remodeler, INO80, in checkpoint maintenance, and (4) to establish the participation of histone chaperons, ASF1 and NASP, in cell cycle checkpoint recovery. Variety of human cell lines lacking individual protein factors, either constitutively or by siRNA/shRNA mediated gene silencing, will be utilized at select stages of cell cycle to analyze the effects on checkpoint protein markers and reveal their functional interactions through FACS analysis, ChIP, co-immunoprecipitation and/or by co-localization assays. Biochemical characterization of ATR/ATM substrates will be achieved by mutational alterations of SQ/TQ substrate motifs followed by their functional analysis. Select histone modifications will be evaluated in specifically compromised cells to reveal alterations regulating NER and cell cycle progression. Lastly, purified recombinant histones and chaperons will be tested in vitro to delineate their NER, checkpoint and cell cycle specific biochemical roles in vivo. These systematic studies will furnish crucial insights regarding the key events initiated upon xenobiotic exposures of mammalian cells with the ultimate goal of human health risk assessment and management.